Sequence-specific hybridization of oligonucleotide probes has long been used as a means for detecting and identifying selected nucleotide sequences, and labeling of such probes with fluorescent labels has provided a relatively sensitive, nonradioactive means for facilitating detection of probe hybridization. Recently developed detection methods employ the process of fluorescence energy transfer (FET) rather than direct detection of fluorescence intensity for detection of probe hybridization. Fluorescence energy transfer occurs between a donor fluorophore and an acceptor dye (which may or may not be a fluorophore) when the absorption spectrum of one (the acceptor) overlaps the emission spectrum of the other (the donor) and the two dyes are in close proximity. Dyes with these properties are referred to as donor/acceptor dye pairs or energy transfer dye pairs. The excited-state energy of the donor fluorophore is transferred by a resonance dipole-induced dipole interaction to the neighboring acceptor. This results in quenching of donor fluorescence. In some cases, if the acceptor is also a fluorophore, the intensity of its fluorescence may be enhanced. The efficiency of energy transfer is highly dependent on the distance between the donor and acceptor, and equations predicting these relationships have been developed by Forster (1948. Ann Phys. 2, 55-75). The distance between donor and acceptor dyes at which energy transfer efficiency is 50% is referred to as the Forster distance (R.sub.o). Other mechanisms of fluorescence quenching are also known including, for example, charge transfer and collisional quenching.
Energy transfer and other mechanisms which rely on the interaction of two dyes in close proximity to produce quenching are an attractive means for detecting or identifying nucleotide sequences, as such assays may be conducted in homogeneous formats. Homogeneous assay formats are simpler than conventional probe hybridization assays which rely on detection of the fluorescence of a single fluorophore label, as heterogeneous assays generally require additional steps to separate hybridized label from free label. Typically, FET and related methods have relied upon monitoring a change in the fluorescence properties of one or both dye labels when they are brought together by the hybridization of two complementary oligonucleotides. In this format, the change in fluorescence properties may be measured as a change in the amount of energy transfer or as a change in the amount of fluorescence quenching, typically indicated as an increase in the fluorescence intensity of one of the dyes. In this way, the nucleotide sequence of interest may be detected without separation of unhybridized and hybridized oligonucleotides. The hybridization may occur between two separate complementary oligonucleotides, one of which is labeled with the donor fluorophore and one of which is labeled with the acceptor. In double-stranded form there is decreased donor fluorescence (increased quenching) and/or increased energy transfer as compared to the single-stranded oligonucleotides. Several formats for FET hybridization assays are reviewed in Nonisotopic DNA Probe Techniques (1992. Academic Press, Inc., pgs. 311-352).
Alternatively, the donor and acceptor may be linked to a single oligonucleotide such that there is a detectable difference in the fluorescence properties of one or both when the oligonucleotide is unhybridized vs. when it is hybridized to its complementary sequence. In this format, donor fluorescence is typically increased and energy transfer/quenching are decreased when the oligonucleotide is hybridized. For example, a self-complementary oligonucleotide labeled at each end may form a hairpin which brings the two fluorophores (i.e., the 5' and 3' ends) into close proximity where energy transfer and quenching can occur. Hybridization of the self-complementary oligonucleotide to its complement on a second oligonucleotide disrupts the hairpin and increases the distance between the two dyes, thus reducing quenching. A disadvantage of the hairpin structure is that it is very stable and conversion to the unquenched, hybridized form is often slow and only moderately favored, resulting in generally poor performance. Tyagi and Kramer (1996. Nature Biotech. 14, 303-308) describe a hairpin labeled as described above with a detector sequence in the loop between the self-complementary arms of the hairpin which form the stem. The base-paired stem must melt in order for the detector sequence to hybridize to the target and cause a reduction in quenching. A "double hairpin" probe and methods of using it are described by B. Bagwell, et al. (1994. Nucl. Acids Res. 22, 2424-2425; U.S. Pat. No. 5,607,834). These structures contain the target binding sequence within the hairpin and therefore involve competitive hybridization between the target and the self-complementary sequences of the hairpin. Bagwell solves the problem of unfavorable hybridization kinetics by destabilizing the hairpin with mismatches, thus favoring hybridization to the target.
Homogeneous methods employing energy transfer or other mechanisms of fluorescence quenching for detection of nucleic acid amplification have also been described. R. Higuchi, et al. (1992. Biotechnology 10, 413-417) disclose methods for detecting DNA amplification in real-time by monitoring increased fluorescence of ethidium bromide as it binds to double-stranded DNA. The sensitivity of this method is limited because binding of the ethidium bromide is not target specific and background amplification products are also detected. L. G. Lee, et al. (1993. Nuc. Acids Res. 21, 3761-3766) disclose a real-time detection method in which a doubly-labeled detector probe is cleaved in a target amplification-specific manner during PCR. The detector probe is hybridized downstream of the amplification primer so that the 5'-3' exonuclease activity of Taq polymerase digests the detector probe, separating two fluorescent dyes which form an energy transfer pair. Fluorescence intensity increases as the probe is cleaved. Published PCT application WO 96/21144 discloses continuous fluorometric assays in which enzyme-mediated cleavage of nucleic acids results in increased fluorescence. Fluorescence energy transfer is suggested for use in the methods, but only in the context of a method employing a single fluorescent label which is quenched by hybridization to the target.
Energy transfer and other fluorescence quenching detection methods have also been applied to detecting a target sequence by hybridization of a specific probe. Japanese Patent No. 93015439 B discloses methods for measuring polynucleotides by hybridizing the single-stranded target to a single-stranded polynucleotide probe tagged with two labels which form an energy transfer pair. The double-stranded hybrid is cleaved between the labels by a restriction enzyme and fluorescence of one of the labels is measured. A shortcoming of this method is that the restriction site in the probe must also be present in the target sequence being detected. S. S. Ghosh, et al. (1994. Nucl. Acids Res. 22, 3155-3159) describe restriction enzyme catalyzed cleavage of fluorophore-labeled oligonucleotides which are analyzed using fluorescence resonance energy transfer. In these assays, the complementary oligonucleotides are hybridized to produce the double-stranded restriction site, and one of the fluorescent labels is linked to each of the two strands. S. P. Lee, et al. (1994. Anal. Biochem. 220, 377-383) describe fluorescence "dequenching" techniques using restriction endonucleases to cleave double-stranded DNA. However, these methods relate to assays employing only a single fluorescent label which is quenched by interaction with the DNA, not by fluorescence energy transfer from a second fluorescent label. Hybridization of the labeled oligonucleotide to its complement and cleavage of the double-stranded restriction site reduced the non-transfer quenching of the label such that quenched fluorescence was totally recovered.
Signal primers (also referred to as detector probes) which hybridize to the target sequence downstream of the hybridization site of the amplification primers have been described for use in detection of nucleic acid amplification (U.S. Pat. No. 5,547,861). The signal primer is extended by the polymerase in a manner similar to extension of the amplification primers. Extension of the amplification primer displaces the extension product of the signal primer in a target amplification-dependent manner, producing a double-stranded secondary amplification product which may be detected as an indication of target amplification. The secondary amplification products generated from signal primers may be detected by means of a variety of labels and reporter groups, restriction sites in the signal primer which are cleaved to produce fragments of a characteristic size, capture groups, and structural features such as triple helices and recognition sites for double-stranded DNA binding proteins. Examples of detection methods for use with signal primers are described in U.S. Pat. No. 5,550,025 (incorporation of lipophilic dyes and restriction sites) and U.S. Pat. No. 5,593,867 (fluorescence polarization detection).